Electrical machine



Oct. 18, 1949. M. LlwscHlTz ErAL 2,484,840

ELECTRICAL MACHINE Filed July 9. 194e 5 sheets-sheet 1 ATTORNEY Oct. 18, 1949. M. LlwscHlTz E1' AL 2,484,840

' ELEGTRICAL MACHINE Filed July 9, 194e s sneetsneet 2 .Z Vf P454 PZ' ATTOR N EY Oct. 18, 1949. M. LlwscHn-z ETAL 2,484,840

ELECTRICAL MACHINE Filed July 9, 1946 5 Sheets-Shea?I 3 /CC/ /CC3 ATTORNEY M. LxwscHrrz E1 AL 2,484,840

ELECTRICAL MACHINE omn 1s, 1949.

Filed July 9, 1946 5 Sheets-Sheet 4 Fig/7,

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ELECTRICAL MACHINE Filed July 9, 1946 5 Sheets-Sheet 5 ZD 264 ZCHM! e ZP@ l ATTORNEY Patented oct. 1s, 1949 ELECTRICAL MACHINE Michael Liwschitz, Brooklyn, N. Y., and Albert W.

Kimball, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 9. 1946, Serial N0. 682,188

Claims. (Cl. 32292) Our invention concerns rotary direct-current machines for generating, controlling or regulating purposes and is related to, and in some aspects an improvement upon, machines of the type disclosed in the copending application Serial No. 607,440, filed July 27, 1945, by W. R. Harding and A. W. Kimball, and assigned to the assignee of the present patent.

It is an object of the invention to achieve within a single-armature machinev an amplifying performance of an extremely high amplification factor in conjunction with high speed of response, satisfactory accuracy and high stability of operation.

To this end, and in accordance with our invention, we provide a multipole direct-current generator with a multistage cascade-type amplification system of field exciting circuits in such a manner that a plurality of amplifier stages receive their respective excitation from separater internal circulating currents. Since the first stage of such a machine is excited by signal, control or input voltage originating from some external source of energy, the invention results in at least three cascade stages of amplification thus affording an overall amplification Whose magnitude, if desired, can be increased to an order of magnitude higher than that obtainable in any single amplifying machine of the rotary type heretofore available. For instance, with an amplification factor of approximately 100 to 1 for each stage, a total amplification in the order of 100 100 100=1,000,000 can be obtained by a three-stage amplifier according to this invention; and a still further increase in order of magnitude can be achieved when applying more than two stages of excitation by internal circulating currents in cascade sequence to the separately excited input stage.

These and other objects and advantages of the invention will be apparent from the following description in conjunction with the drawings in which:

Figure 1 shows diagrammatically the field structure, commutator and the signal field windings of a four-pole machine according to the invention;

Fig. 2 is a schematic showing of the same machine and illustrates the control and forcing field windings under omission of the field-pole structure;

Figure 3 represents the same circuits as Fig. 2 in simplified straight-line fashion;

Fig. 4 is an explanatory and symbolic diagram representing the three amplifying stages involved in the machine according to the preceding iigures;

Figs. 5 through 12 are additional diagrams explanatory of the performance of the Asame machine;

Fig. 13 is a circuit diagram, also appertaining to the machine shown in the preceding figures, and shows compensating field windings in addition to those aforementioned;

Fig. 14 shows the circuits of the commutation windings for the interpoles of four pole machines according to the invention;

Fig. 15 is the complete circuit diagram of a three-stage four-pole amplifying machine embodying the details shown in the preceding gures;

Fig. 16 illustrates the stator frame and armature structure of an eight-pole machine, while Figs. 17 and 18 represent the circuit diagrams of two respective embodiments containing such an eight-pole structure;

Figs. 19 and 20 are circuit diagrams of fourpole three-stage machines according to the invention in which one of the amplifying stages is excited by armature reaction; and

Fig. 21 shows the circuit diagram of an eightpole machine also embodying the use of armature reaction for the excitation of internal amplifying stages.

According to Figure 1, the field structure F of the machine is designed like that of a conventional four-pole generator. It has four main poles PI, P2, P3, P4 and four interpoles QI, Q2, Q3, Q4. The armature A has a commutator with four brushes BI, B2, B3, B4. The armature conductors (not shown) are lap wound with conventional four-pole chording. As regards its circuit connections, of course, the machine differs fundamentally from those of `normal generators. For the sake of explanation, however, the output (load) circuit and all internally excited machine circuits to be referred to below, are omitted in Fig. 1 which shows only the separately excited input circuit extending between the input terminals Ap and An. This circuit includes two field windings SI and S3 disposed on poles PI and P3, respectively. Coils Si and S3, hereinafter called control coils, are so connected that the small signal voltage applied between the primary terminals Ap and An magnetize poles Pl and P3 in opposite directions to produce north polarity in one pole and south polarity in the other.

Fig. 2 shows the same machine more schematically as regards structural features but indicates all main field windings as well as their relative positions on the main pole axes Pl--Pl and PZ-PI. also represented in Fig. 3 by a diagram which discards the showing of spacial relations between the field coils in favor of a simplified and clarified representation of the basic electric circuit connections of the machine. Both figures show the control coils Sl and S3 connected between input terminals Ap and An and located on poles PI and P3, respectively, in accordance with Fi 1.

igNhen the control coils SI and S3 are excited by signal voltage, with the armature of the machine running clockwise, the control flux (oc) caused by the signal excitation induces a voltage Vi (Fig. 3) between brushes BI and B3. Assuming the control flux pc so directed that it causes north polarity in pole Pl and south polarity in pole P3, brush BI will be negative and brush B3 will be positive as regards the induced voltage. According to Figs. 2 and 3, brushes Bl and B3 are interconnected by a circuit Cl which includes four series-connected eld windings (forcing coils) iC2, IC4, |D2, ID4, of which those denoted by iC2 and |D2 are located on pole P2 while windings lC4 and |D4 are associated with pole P4. The voltage Vi generated between brushes BI and B3 drives an internal circulating current Il (Fig. 3) through circuit Ci. This current energizes coils iC2 and iD2 cumulatively so that pole P2 is excited by the sum of ampere turns of both coils. Similarly, pole P4 becomes cumulatively excited by coils iC4, iD4 and assumes a magnetic polarity opposite of that of pole P2 (Figs. 1, 2).

This excitation of poles P2, P4 induces in the armature another voltage (V2) between brushes B2 and B4. A circuit C2 extends between brushes B2 and B4, and includes eight seriesconnected field windings (forcing coils) ZCI, 2C2, 2C3, 2C4, 2Dl, 2D2, 2D3, 2D4. Coils 2Cl and 2Dl are located on pole Pi and are cumulatively energized by the internal circulating current I2. driven through the circuit C2 by voltage V2 (Fig. 3). Similarly, coils 2C2 and 2D2 are disposed on pole P2, coils 2C3 and 2D3 on pole P3, and coils 2C4 and 2D4 on pole P4, each pair being cumulatively energized by current I2. As a result, all four main poles PI, P2, P3, P4 become excited by the voltage V2 between brushes B2 and B4. This excitation, taken by itself, is symmertical and balanced; that is, it tends to impart equal strengths to the four poles and causes opposite poles to assume opposite magnetic polarities thus making poles PI, P3 north poles and poles P2, P4 south poles.

The secondary or load terminals Tp and Tn of the machine (Figs. 2, 3) are connected to the mid-points Mi and M2 of circuits CI and C2, respectively. Hence, terminal Tn is in connection with brushes Bi and B3, while terminal Tp is connected with brushes B2 and B4.

The above-described machine operates as a triple-stage amplifier so that the secondary or output voltage appearing across the load terminals Tp and Tn varies in dependence upon the changes of an input or signal -voltage applied across terminals Ap and An. The three stages of amplication are more readily apparent from the equivalent substitute diagram of three cascade-connected machines shown in Fig. 4. In this explanatory diagram, three separate machines or armatures Al, A2, A3 are shown instead of the single armature actually used, and the above-described field windings, circuits and All field windings shown in Fig. 2 are 4 terminals are accordingly represented by the electric arrangements that they would have if three separate machines were used instead of the one according to the invention.

The first stage of amplification receives separate input excitation from terminals Ap and An and may be considered to represent a twopole machine whose poles Pl and PI (Fig. l) are energized by the control coils Si and SI. The amplified output voltage (VI) of this stage is generated in the armature windings between brushes Bl and B3.

The second amplifying stage may be considered to represent a second two-pole machine (poles P2 and P4) which receives input or field excitation from the brushes Bi and B3 of the first two-pole machine. This input excitation is applied by circuit CI to the field coils (forcing coils) 2C2, 2C4, 2D2, 2D4 on the poles P2. P4 of the second two-pole machine. The amplified output voltage (V2) of the second stage appears between the brushes B2 and B4 of the second machine.

The third stage of amplification may -be considered to involve a four-pole machine including all four poles and all four brushes of the generator. The input circuit C2 of this four-pole machine is excited by the output voltage (V2) between the brushes B2. B4 of the second twopole machine, while the terminals Tn and Tp of the output circuit are attached to an equalizer connection C2 between brushes B2, B4 and another equalizer connection CI between brushes Bi and B3.

While instead of three separate machines shown in the schematic diagram of Fig. 4, the three stages of amplifiers according to the invention are superimposed and merged into a single machine, the phenomena occurring in such an amplifier, as far as they' are intended to be represented by Fig. 4, can, in fact, be considered separately, provided the magnetic structure of the machine operates within the unsaturated region of its magnetization curve. Then, the magnetic fluxes occurring in the above-assumed hypothetical three machines superimpose themselves upon one another substantially as if they occurred in separate magnetic devices. Hence, a rating of the magnetizable machine elements for operation below saturation is an essential requirement of the invention as regards all embodiments and modifications described in this specification. A machine thus designed is in fact equivalent to three cascade-connected machines as regards the presence of three amplifying stages. However, since in reality a machine according to the invention has only one frame and armature structure, it affords not only considerable saving in material and space but avoids also the magnetic sluggishness inherent in the performance of three cascade machines. In other words, an amplifier according to the invention has a much lower time constant, i. e.. a highly increased speed of response of its amplified output voltage to changes in signal voltage than could possibly be obtained with three separate machines.

The performance of the above-described fourpole amplifier (Figs. 1 to 3), involving a superposition of three generators within a single machine, will be more fully understood from the detailed discussion of the magnetic and electric characteristics presented below with reference to the explanatory diagrams of Figs. 5 to 11. It should be noted that in these figures, the brushes BI, B2, B3, B4 are shown not in their actual positions but, for the purpose of explaining relations, are illustrated without a-oommutator as if they contacted directly the armature conductors at the proper points of commutation.

It will be noted, from a comparison of the threemachine scheme of Fig. 4 with the single machine of Figs. 1 to 3. that in a single machine according -cults (C2, CI) of the preceding stages. It has been explained above that, with magnetization below saturation, the three stages operate as if they were separate devices in magnetic respect.

It remains to be discussed, however, why the com'- munity of brushes and circuits of the several stages does not interfere with the desired triplestage amplification.

Fig. shows the main poles PI, P2, P3, P4 and brushes BI, B2, B3, B4 of the last amplifying stage representing a four-pole machine. Considering merely the four-pole excitation fuirn'ished by coils 3CI', SC2, SC3, 304, SDI, 3D2, 3D3, 3D4 (Figs. 2, 3, 4) poles Pl and P3 are north poles and poles P2 and'Pl are south poles and provide a magnetization of symmetrical ilux distribution. For clockwise rotation, brushes BI and B3 are negative, and brushes B2 and B4 are positive. With respect to this four-pole machine, the lap-wound armature windings are normal, that is, they have little or substantially no chording as in conventional four-pole generators. Hence, the angular position of the brush axes .Bl-B3 and B2-B4 at which the brushes pick up maximum voltage are substantially 45 displaced from the pole axes Pl--P3 and P2-P4.

Referring now to the two two-pole machines superimposed on the four-pole machine and forming respectively the rst and second stages of the cascade, the axes of the respective pairs of brushes would have to be 90 displaced from the pole axes if the armature windings had no appreciable chording. As regards these two-pole machines, however, the armature windings are not normal but have 50% (90) chording. Fig. 6 shows schematically a two-pole machine with north (N) and south (S) poles, brushes Ba and Bb, and chorded armature windings, such as those represented by WI and W2. It holds good, in general, that when the chording of the armature windings in a two-pole machine is equal to the position of the brush axis Ba-Bb, for maximum voltage between the brushes, is shifted by the angle /2 opposite to the chording angle regardless of the polarity of the poles and the direction of armature rotation. Consequently, in the ilrst two-pole stage of the machine, the maxlmum-voltage position of the appertainlng brushes BI and B3, as shown in Fig. 7, is 45 displaced from the pole axis PI-P3 and therefore coincides 4with the maximum-voltage lposition of these brushes in the four-pole stage (Fig. 5). Correspondingly, the position of brushes B2 and B4, appertaining to the second two-pole stage, when adjusted for maximum voltage is identical with the maximum voltage position of the same brushes relative to the four-pole stage.

While relative to the four-pole stage, poles Pl and P3 are both north poles and brushes BI and B3 are both electrically negative, the magnetic and electric polarities are ldifferent for the same poles and brushes with respect to the ilrst twopole stage. Under the above-mentioned operating conditions and as shown in Fig. 7, pole Pl is I negative andbrush Blto be positive.\1fit is kept w in mind that the magnetic. nuxandelectric voltage vare strongest in the four-'pole singeY of the-g amplifying system, it will be that -the "'S-nblivole (n) ndple Psa south pole (a.) f inthistwo-pole stage, `and the oo" magnetic control ilux pc causesbrush BI. to be magneticrand electric conditions of the first two- Y pole stage. superimposed on the `fourpole stage',` I l havethe eil'ect of strengthening the north. po-

larity in pole `PI of the four-pole machine (N--n) while weakening the'north. polarity of pole P3 (N-s), and that similarly the brush BI of the four-pole stage becomes more strongly` negative while the negative' potential of brush B3 is reduced. Thus. the performance of the first two-pole stage has the eiiect of distorting the flux distribution of the normally symmetrically excited iour-pole machine and imposes a controlled difference in potential on the interconnected equipolar (negative) brushes BI and B3 of the four-pole machine. The current (Il, see Fig. 3) owlng between brushes BI and B3 through the appertaining equalizing connection CI may therefore be considered to be caused by the distortion or asymmetry of flux distribution imposed on the four-pole machine.

Similarly, the second two-pole stage, as represented in Fig. 8, has its north pole (n) at pole P2 and its south pole (s) at pole P4. Brush B2, relative to this two-pole stage, assumes negative polarity while brush B4 becomes positive. Superimposed on the predominant excitation of the four-pole stage, the resultant effects are a weakening of south pole P2 and strengthening oi' south pole P4 of the four-pole machine thus causing another distortion of the normally balanced four-pole flux. The equipolar brushes B2 and B4 of the four-pole machine, in further consequence, assume different potentials, brush B2 reducing and brush B4 increasing their respective positive values. Hence, the circulating current I2 (Fig. 3) may also be considered to be an equalizing current resulting between equipolar brushes of a four-pole machine due to a controlled distortion or unbalance of the four-pole flux distribution.

The flux pc (Fig. 7) produced in the rst twopole stage by the signal voltage imposed on control coils Si and S2, while controlling the voltage (VI) between brushes Bl and B3, does not induce any voltage between brushes B2 and B4; conversely, the current Il (Fig. 3) which is the exciting current of the second stage generates the voltage V2 between brushes B2 and B4 under control by forcing coils |C2, IC4, ID2, IDI and does not cause any voltage between brushes Bl and B4; nor has the output current of the fourpole stage, appearing between terminals Tp and Tn, an influence on the voltages Vl and V2 of the preceding two two-pole stages. This will be understood from the following considerations.

Fig. 9 elucidates the electromotoric forces induced by the control ux c in the iirst two-pole stage between the brushes Bl and B3, and Fig. 10 refers to the electromotoric forces caused by the same ilux between the brushes B2 and B4. Only one of the two armature paths between each brush pair is considered in these figures. A number of conductors, arranged in outer and inner layers, are represented. Those denoted by a cross (X) are induced so as to pass current in a direction from the observer toward the plane of illustration, and those denoted by a dot are induced to pass current in the direction from the plane of illustration toward the observer. When the armature rotates clockwise and the control coils are excited to induce control flux rpc, the conductors of the two-pole amature which lie in the interpolar space between brush B3 and position a (Fig. 9) contribute nothing to the voltage between Bi and B3 because their E. M. F.s

cancel each other. The resultant voltage between brushes BI and B3 is composed of the E. M. F.s in the outer conductors that lie between Bi and a, and those of the inner conductors which lie between B3 and b. The E. M. F.s in the inner conductors have a direction opposite to that of the outer conductors so that the effective E. M. F.s of both conductor layers are additive. Referring now to Fig. 10, it will be seen that relative to brushes B2 and B4, the conductors lying between B2--a and b-B4 can be disregarded. In the remaining conductors between b' and B2, the E. M. F.s of the inner layer have the same direction as those of the outer layer so thatthey cancel each other. Hence no resultant voltage appears between brushes B2 and B4 as a result of flux c. It should be noted that the flux c, passing through the poles Pl and P3, does not traverse the poles P2 and P4, as is schematically indicated in the diagram of Fig. 11.

A similar consideration, applied to the second two-pole stage, shows that the amplified flux pi produced in poles P2 and P4 by the current Il (Fig. 3) controls only the voltage V2 between brushes B2 and B4 but has, in fact, no effect on the voltage between brushes BI and B3. The flux pi through poles P2 and P4 does not pass through the poles Pi and P3, as is also represented in Fig. 11.

The foregoing discussion based on Figs. 9 and 10 can be summarized by stating that only one of the two brush pairs Bi, B3 or B2, B4 can possibly derive voltage from the appertaining twopole machine, because, `since that one pair is adjusted to the maximum voltage of the machine, the other pair must necessarily be adjusted to zero voltage as it is 90" displaced from the maximum voltage position.

When a load is connected to the secondary terminals Tp and Tn, the load current Is (Fig. 3), though flowing through the circuits Cl and C2, should not influence the above-mentioned voltage conditions of the two-pole stages. The design and arrangement of the forcing coils in pairs, such as the coil pair IC2 and ID2 on pole P2 or the pair 2C| and 2Di on pole I, serves to obtain such load independence. The coils of each pair have equal turns and are so arranged in the respective circuits Cl and C2 that they lie on opposite sides of the mid-point MI or M2. Therefore, the load current Is, flowing from terminal Tn to point MI (Fig. 3) divides itself into two components which, in circuit CI, traverse the two parallel paths Mi-iC4-lC2-BI and Mi-iD2-|D4-B3, The two load current components thus energize the coils IC2 and ID2 in opposite directions so that their resultant effect on pole P2 is zero. For the same reason, the coils lCd and ID4 on pole P4 cancel each other as regards the magnetizing effect of the load current. In other words, while the coils of each pair act cumulatively on the appertaining pole as regards the effect of the internal circulating current Il, they are differentially connected and mutually balanced relative to the flow of load current.

The coils of pairs 2C|2Di, 2C2-2D2, 2C3-2D3, and 2C4-2D4 in circuit C2 are also disposed in a balanced arrangement relative to the point M2. Hence the load current Is, flowing in two parallel brances from B2 to M2 and from B4 to M2 has again no effect on the magnetization of the four-pole stage of the machine.

The flow of current in the armature conductors of machines according to the invention has the effect of producing an armature reaction flux. As regards the armature reaction secondarily caused by the control field excitation (control coils Si and S2) of the first two-pole stage and the armature reaction incident to the operation of the four-pole stage, the reaction effects are not different from those occurring in conventional generators. That is, these reaction phenomena can readily be compensated by a proper rating of the forcing coils whose efficacy the phenomena tend to reduce. The armature reaction due to the operation of forcing coils IC2, ID2 and IC4 and ID4 on poles P2 and P4 of the second amplifying stage, however, should be given special consideration in order to increase the efficiency and sensitivity of the machine beyond the performance values otherwise attainable. This will be understood from the following reference to Fig. 12.

As shown schematically in Fig. 12, the exciting flux I of the second two-pole stage, produced by the forcing coils on poles P2 and P4 and inducing voltage in the armature conductors between brushes B2 and B4, is also the cause of a secondary armature reaction flux pa which extends at a right angle to the pole axis P2-P4, This reaction flux pa is in opposition to the control fiux oc caused by the excitation of the ccntrol coils SI and S2 of the first two-Dole stage and hence tends to weaken the primary control fiux. If no provision is made to reduce or eliminate this weakening effect, the machine will tend to operate under such conditions that the reaction flux aa is smaller than the control flux pc by just a sufficient amount to maintain a flow of circulating currents Il and I2 (see Fig. 3); and this would result in a rather inefficient use of the control field energy and put a corresponding limit to the use of the machine for amplifying extremely minute signal voltages.

It is, therefore, preferable to provide the machine with compensating coils which balance the demagnetizing armature reaction flux a of the second two-pole stage as exactly as possible. Such windings may be either of distributed or concentrated type, although only the latter type, for convenience, is shown in the following illustrations. These compensating coils may be placed on the pole shoes of poles PI and P3 and energized by the circulating current Ii or I2 or both of the internally excited two pole stages. Accordingly, Fig. 13 shows a circuit diagram, similar to that of Fig. 3, which includes compensating coils ICCi, ICDI on pole Pi, and coil ICC3, iCD3 on pole P3. These coils are connected in circuit CI to be energized by the internal circulating current Ii. Fig. 13 shows further a group of compensating coils 2CCI, '2CDI and 2CC3, 2CD3 for poles Pi and P3 respectively which are connected in circuit C`2 and energized by the circulating current I2. Either one or both of these groups may be employed, and in each case be given the number of turns required for the desired degree of compensation of fiux a (Fig. 12). The two compensating coils on each pole, such as coils ICCI and ICDI on pole Pi, are so connected that they act commulatively relative to the energizing circulating current but are differential and balanced with respect to the load current Is of the four-pole stage. Consequently, the compensating effect is not influenced by the load current oi' the machine.

In order to secure proper commutation, machines according to the invention, like ordinary generators, are to be provided with commutation or interpole windings located on the interpoles QI, Q2, Q3 and Q4 of the field structure shown in Fig. 1. In order to obtain optimum performance, however, design and connection of the commutatlng field windings are preferably different from convention-al generators. While in ordinary generators, the current distribution within the armature conductors between the load brushes is normally balanced, these currents do not maintain a fixed ratio of distribution in machines according to the invention. The distribution is substantially balanced only when the signal excitation of the control coils (Si and S3 in Figs. 1 to 3) is zero. With a finite excitation applied to the control coils, however, the circulating. or unbalance currents Ii and I2 (see Fig. 3) will flow between brushes IBI- B2 and B2-B4 respectively, and these unbalance currents vary in magnitude depending upon the degree of signal excitation. Hence, the commutating coils on lnterpoles QI, Q2, Q3, Q4 should be designed to take care of proper commutation `not only relative to the balanced current distribution of the four-pole stage but also with respect to the unbalance conditions caused by the presence of the' circulating currents Il and I2. The following consideration leads to a more accurate expression of the requirements to be met by the interpole coils and refers to machines whose detrimental armature reaction is compensated by concentrated coils (rather than by distributed windings in the pole shoes).

If N represents the total number of turns of the armature, Is the load current in the secondary circuit Cs, Il the unbalance current in connection CI between-brushes BI and B3, and I2 the unbalance current in connection C2 between brushes B2 and B4, the total effective turns (Ni of the commutation coils on the interpole axis Qi-Q3 required to commutate the currents Il and I2 is:

N1---I1+I2 (l) and the number of effective turns (N2) on the axis Q2-Q4 for commutating the currents Il and 'I2 is:

For the commutation of the load current Is each interpole requires the number (N3) of turns:

N h g NS-x- 8l (3) because is the component load current flowing in each of the two parallel paths formed by each of the connections Ci and C2.

Thus the interpole windings on axis QI-QB must satisfy the Equation 1 for the unbalance currents and also the Equation 3 for the load current; while the windings on axis Q2-Q4 must be in accordance with Equation 2 for the unbalance currents and also with Equation 3 for the load current.

An example of interpole coils satisfying these A conditions for the approximation that the current value oi Il and I2 are equal, is illustrated in Fig. 14. The interpole QI (see Fig. 1) is provided with four commutation coils IQI, 2QI, IQI, 4QI; interpole Q2 has four commutation coils iQ2, 2Q2, 3Q2, 4Q2; interpole Q3 has four coils IQ3, 2Q3, JQI, IQI; and the four commutation coils IQ4, 2Q4, 3Q4, 4Q4 are disposed on interpole Q4. The commutation coils whose designation includes the prefix 1 are series-connected between brush BI and a point Bi' in the circuit C I between brushes BI and B3. as is more clearly apparent from the complete circuit diagram shown in Fig. 15. The commutation coils prefixed by 3 lie also in series between brush B3 and point B3' in circuit Cl. The commutation coils prefixed by 2 and 4 are series-connected between B2, B2' and B4, B4', respectively, in the circuit C2 between brushes B2 and B4.

As schematically shown in Fig. 14, two of the commutation coils on each interpole have more turns than the other two. The two coils of each interpole shown next to the armature have three times as many turns as the remaining coils. Under no-load condition, with the control fields effective to produce a flow of circulatingeurrents Ii and I2 through the commutation coils, as represented in Fig. 14 by full-line arrows, the resultant effective number of turns (N2, see Equation 2) of all Q2 and Q4 coils is zero. The turns of the QI coil and Q3 coils, however, act cumulatively so that the resultant number (N i, see Equation l) of effective turns in the QI-Q3 axis is equal to the sum of the turns of all QI and Q3 windings. In the opposite extreme as represented by broken-line arrows, i. e., with zero control field and a finite load current, normal interpole strength and polarity is obtained because the two outer coils on each interpole act differentially with respect to the two inner coils thus producing the correct net turns (N3) needed in accordance with Equation 3 (for Ii=I2). Under intermediate conditions, the two effects represented in Fig. 14 combine to produce the desired commutating fields.

The performance of interpole coils as described above can be further improved by giving the larger coils (iQi, 4Qi, 2Q2, iQ2, 3Q3, 2Q3, 4Q4. 3Q4) somewhat more than three times the turns of the other interpole coils so that the additional turns will take care of the electromotive forces of self-inductance in the armature windings that are short-circuited lby the commutator brushes.

Machines according to the invention, as described so far, operate exclusively as an amplifying device; that is, the occurrence and value of the output voltage depends only on the presence and value of the signal voltage. However, it is also possible to design such a machine for operating it as a normal generator when the excitation of the control field is zero, and asan amplifier superimposed on a normal generator when the control fleld is energized. The machine will then provide a given finite output voltage at zero control excitation and will vary or regulate this output voltage when a control excitation is applied. The above-exemplified four-pole machine,for instance, can be operated as a normal series generator if, in departure from the foregoing description of a purely amplifying device, the two coils appertainlng to each pair of forcing coils of the four-pole stage are given unequal numbers of turns so that their differential excitation due to the load current Is is not balanced. For instance, the coils 2CI, 2C2, 2C3 and 2C4 (see Fig. 3 or 15) may be given more turns than the coils 2DI, 2D2, 2D3 and 2D4. A differential excitation of finite and variable magnitude is then effective in the four-pole machine wherein a load is applied to the output terminals Tp and Tn. This excitation is equivalent to a series-excitation and dependent upon the magnitude of the load current. Instead of such a design, or together therewith, a series coil may be placed on each of the four poles of the machine and connected in the secondary circuit of the machine such as the eld coils FI, F2, F3, F4 shown in Fig. 15. Shunt field windings, as denoted in Fig. by GI, G2, G3, G4 on poles PI, P2, P3, P4 respectively may also be used. As a matter of fact, the use of weak shunt fields connected acrossv the load terminals may be. useful for machines, amplifiers or other generators, in order to afford a finer adjustment, i. e., for securing an operation of the machine on the straight portion of its magnetic characteristic.

Secondarily excited series and shunt field windings, arranged as shown in Fig. 15, may also be used to obtain tuned feedback excitation. To this end, the resistance of the secondary circuit including the coils is so adjusted, by a proper rating of the coils and, if desired, by means of a calibrating rheostat such as the one denoted by Rh in Fig. 15, that the resistance line coincides approximately with the no-lead characteristic of the machine. Thus tuned, the fields of these load-circuit-excited coils provide substantially all excitation needed to maintain the output voltage of the machine at any value along the straight portion of the characteristic, so that the signal and forcing fields serve merely to shift this voltage along the characteristic to any point or value dictated by the signal excitation.

The circuit diagram of Fig. 15 illustrates also a modification of the signal-responsive input stage of the machine. While according to Figs. 1 to 3, and 13, a single primary circuit is used to provide control excitation for the input stage, Fig. 15 shows a circuit with terminals Bp, Bn and coils RI, R3 in addition to the signal circuit Ap-SI-S3-An. Coil RI is located on pole PI and opposes coil Sl. Coil R3 is located on pole P3 to oppose coil S3. When the generator is in operation, a pattern voltage of selected magnitude is applied to terminals Bp and Bn, and the voltage (pilot voltage) to be responded to is applied to terminals An and Ap. When the pilot voltage equals the predetermined value of the pattern voltage, the iields of coils SI and RI on pole Pl balance each other, and the fields S2 and R2 on pole P3 are also in balance so that the resultant control field is zero. When the pilot voltage departs from the pattern value, a resultant differential control field is produced in accordance with the magnitude of the departure.

In all other respects, the machine represented In Fig. 15 involves the features discussed previously in conjunction with Figs. 1 through 14 as will be recognized from the use of corresponding reference characters. Thus, coils |C2, IC4, ID2, ID4 are the forcing coils of the second two-pole stage energized by the circulating current flowing between brushes BI and B3 in circuit CI; and coils 2CI, 2C2, 2C3, 2C4, 2DI, 2D2, 2D3, 2D4 are the forcing coils of the four-pole stage energized in circuit C2 by the circulating current between 12 brushes B2 and B4. The coils whose reference characters contain the letter Q are interpole windings and designed as explained above in conjunction with Figs. 14 and 15.

Figs. 16 to 18 illustrate the application of the invention to eight-pole machines. The magnetic frame or stator structure F' of such machines, as diagrammatically shown in Fig. 16, is similar to that of conventional eight-pole generators. The appertaining poles are denoted by PI through P8. The armature conductors (not shown) have normal, i. e.. little or substantially no chording relative to the eight-pole structure. The commutator has eight brushes, denoted by BI through B8, which are positioned substantially as normal for ordinary eight-pole generators.

One of the possible circuit diagrams for an eight-pole machine according to the invention is shown in Fig. 17. Four control coils SI, S3, S5, Sl are series-connected between input terminals Ap and An and are disposed on poles PI, P2, P3, Pl respectively. When the armature runs clockwise, pole PI being a north pole, P2 a south pole, P3 a north pole and so forth (relative to the eight-pole output stage), the brushes BI, B3, B5, Bl have negative potentials of the same magnitude, and brushes B2, B4, B6, B8 have positive potentials of the same magnitude as long as no signal voltage is applied. The occurrence of a signal voltage across terminals Ap and An causes coils SI and S3 to strengthen equally the respective south poles PI and P3, while coils S5 and Sl weaken the south poles P5 and Pl in equal amounts. As a result, brushes BI and B3 increase this negative potential and brushes B5 and B'I decrease their negative potential. That is, the potentials of equlpolar brushes BI and B3 remain substantially equal to each other, and the potential of equipolar brushes B5 and Bl remain also equal to each other. Hence, no equalizing current flows in connection Enl between brushes BI and B3, and in connection E112 between brushes B5 and Bl. The circuit CI extending between the connections Eni and En2 is traversed by an internal circulating current II flowing from both brushes B5 and Bl to both brushes BI and B3. This current II energizes eight forcing coils IC2, IC4, ICS, ICB, ID2, ID4, IDB, IDB which are located on poles P2, P4, P8 and P8 so that coils IC2 and ID2, IC4 and ID4, ICS and IDB, IC8 and IDB act cumulatively relative to current II. The excitation .of poles P2, P4, P6 and P8, controlled by the forcing coils of circuit CI, causes brushes B6 and B8 to equally increase, and brushes B2 and B4 to equally decrease their respective positive potentials. Equipotential brushes BB and B8 are interconnected by an equalizing conductor Epl, and another equalizing conductor Ep2 is connected between brushes B2 and B4. The circuit C2, attached across the connections Epi and Ep2, is traversed by an internal circulating current I2 which flows from brushes B6 and B8 to brushes B2 and B4. Eight forcing coils denoted as a whole by 2CI to 2C8 and eight forcing coils denoted by 2DI to ZDB are all series connected in circuit C2 and disposed in pairs on all eight poles. The output terminals Tp and Tn of the machine are connected to midpoints M2 and MI of circuits C2 and CI respectively. The arrangement and rating of the C" coils and D coils is such that the load current Is has no magnetizing effect on the poles (unless a series excitation is desired) as explained above with reference to the four-pole machine,

The eight-pole generator just describes' repre 13 sents a three-stage amplifier, the three stages having a pole ratio of 4:4:8. The ilrst stage is separately excited by control coils Sl, SI, Sl, S1 on the four poles PI, P3, P5, P1 and provides an amplified voltage between brushes BI, B2 and B5. B1. The second stage has also four poles. Its input circuit CI includes the forcing coils IC2, IC4. ICG, ICB. ID2, IDI, IDB, ID3 on poles P2, P4, P6, P8; and its output voltage appears at brushes B2, B4, B6, B8 across circuit C2. The last stage includes all eight poles with a total of sixteen forcing coils marked 2CI to 2GB," and 2DI to 2D8. The output circuit between secondary terminals Tp and Tn, involves all eight brushes of the machine.

It will be recognized that this ampliiier may be looked upon as being substantially a duplication, within a single eight-pole machine, of the four-pole three-stage amplifier described previously. It is an outstanding feature of the invention, however, that it permits increasing the number of amplifying stages for any selected total pole number up to the mathematically possible maximum if desired. Thus, forv instance, an eight-pole machine, having a stator and armature structure as shown in Fig. 16, may be given any number of amplifying stages up to the maximum of seven cascades. The invention makes it further possible to provide plural-stage amplifiers in which the pole ratio between one stage, preferably the last one, and the preceding stage is larger than 2:1. This will be understood from the basic circuits .of the four-stage ampiier described below with reference to Fig. 18.

The eight-pole machine schematically represented by Fig. 18 has four amplifying stages of a pole ratio 2:2:2:4. The signal voltage is applied to input terminals Ap and An and energizes control coils SI and S on the vfirst and fifth poles (north poles) causing a difference in potential to appear between (negative) brushes BI and B5, thus driving a circulating current through the input circuit CI of the second stage. The second two-pole stage is excited by forcing coils I C3, ID3 on the third pole and forcing coils IC1, ID1 on the seventh pole. The output voltage .of this second stage appears across brushes B3 and B1 and drives a circulating current through the input circuit C2 of the third twopole stage. Circuit C2 includes forcing coils 2C2 and 2D2 on the second pole and forcing coils 206, ZDB on the sixth pole and induces ay voltage across brushes B2 and B6. The latter voltage excites the input circuit C3 of the fourth two-pole stage with forcing coils 3C4 and 3DG on the fourth pole and forcing coils SC3 and 3DS on the eighth pole. The output voltage of the third stage appears across brushes B4 and B3 and excites the input circuit C4 .of the last stage which has forcing coils 4CI to C8 and DI The output circuit of the last (eight pole) stage extends from secondary terminal Tn to mid-points MI and M2 and thence to all negative brushes BI, B5, B3, B1, while secondary terminal Tp is connected through mid-points M3 and M4 to all positive brushes B2, B6, B4, B8.

In all embodiments of the invention described so far, the useful field excitation for energizing all amplifying stages of the respective machines is induced by field coils. It should be understood, however, that the invention permits also the application of armature reaction ux for providing the exciting field for one or several stages oi the amplifying machine system. This is explained below with reference to the examples of an armature-reaction excited four-pole machine (Figs. 19, 20) and an armature-reaction excited eightpole machine, Fig. 21).

I'he tour-pole generator shown in, Fig. 19 in analogy to the completeLv coil-excited machine represented by Figs. 2 and 3, has a normal magnetic stator structure and a four-pole armature with four brushes as shown in Fig. 1. The externally excited input circuit of the first amplifying stage has two control coils SI and S3 disposed on poles PI and P3 and connected to input terminals Ap and An. also as shown in Fig. 1. In contrast to the machine according to Figs. 2 and 3, however, the internal circuit CI in the generator of Fig. 19 contains no forcing coils and consists of a short-circuit between brushes BI and B3. Hence, the voltage difference caused between brushes BI and B3, due to the control field c of coils SI and S3, drives a rather heavy current II through the connection CI and through the armature windings short-circuited thereby. The armature windings produce an armature reaction flux which extends at a right angle to the control flux and passes through poles P2 and Pl thereby exciting these poles. As a result, a voltage difference is produced between equipolar brushes B2 and B4, and this voltage difference is applied, in circuit C2, to pairs of forcing coils ZCI and 2DI, 202 and 2D2, 2C3 and 2D3, 2Cl and 2D! disposed on all four poles respectively. The secondary terminals Tn and Tp are attached to the short-circuit connection CI and to the midpoint MI of coil circuit C2 respectively. It will be recognized that this machine is a three-stage amplifier with a pole ratio of 2:2:4 whose input circuit is externally excited by signal voltage while the remaining two stages are excited by internal circulating currents. Aside from the utilization of armature reaction for exciting the second twopole stage, the performance of the machine is similar to that of the coil-excited machine first described.

In machines according to Fig. 19, the current I2 flowing between brushes B2 and B4 induces an armature reaction flux in opposition to the control flux qsc of poles Pi and P3. For optimum results, therefore, this demagnetizing reaction flux must be reduced or compensated by compensating coils substantially in the manner already described. Fig. 20, for instance, shows the circuit diagram oi' a machine, otherwise similar to that of Fig. 19, which has pairs of compensating coils 2CCI, 2CDI on pole PI and 2CC3, 2CD4 on pole P3. These coils are traversed by the current I2 which causes the demagnetizing flux and are so connected as to reduce the flux. The coils of each pair act cumulatively relative to current I2 but are differential and balanced with respect to the load current. Interpole windings may be provided in accordance with Fig. 14, and all other modifications described above may likewise be applied to machines of the armature-reaction excited type.

The eight-pole generator represented, in principle, by the circuit diagram of Fig. 2l has four amplifying stages of a pole ratio 2:2:4:8 and is equipped with a frame and armature structure of the type shown -in Fig. 16. The control coils SI and S5, connected to primary terminals Ap and An, are disposed on poles PI and P5 and induce a voltage difference between negative brushes BI and B5 which are short-circuited by circuit CI. The short-circuit current circulating through the armature windings produces an armature reaction nux in the pole axis P3-P1 so that a corfifteen@A responding voltage difference is produced between negative brushes B3 and B1, which, in circuit C2, excites pairs of forcing coils 2C2, 2D2 and 2C4, 2D4 and 2C6, 2D6 and 2C8, 2D8 on poles P2, P4, P6, P8 respectively. Positive brushes B2 and B6 are interconnected by an equalizing connection Epi, and positive brushes B4, B8 are interconnected by a connection Ep2. The voltage difference between connections Epl and E172 is applied by circuit C3 to eight forcing coils denoted by 3Cl to 308 and eight forcing coils denoted by 3DI to 3D8. These coils are arranged in pairs on all eight poles and provide excitation for the last amplifying stage. The secondary terminal Tp is attached to the mid-point M3 of circuit C3 and thus in connection with all positive brushes B2, B6, B4, B8. The secondary terminal Tn is attached to connection CI and to the midpoint M2 of circuit C2 and thus connected to all negative brushes BI, B5, B3, B1. Interpole windings and coils for compensating demagnetizing armature reaction in the several circuits may be provided in accordance with the above-explained principles but are not shown in the basic diagram of Fig. 21.

It will be obvious from the various modifications referred to in the foregoing that as regards the total number of machine poles, the number, circuit arrangement and pole ratio of the plurality of amplifying stages incorporated in the machine as well as regards such auxiliary means as compensating windings and interpole windings, the invention permits of many variations and changes without departing from its principles and within the scope of its essential features as set forth in the claims attached hereto.

We claim as our invention:

1. A rotary direct-current generator, comprising a field structure with a plurality of poles; an armature having a commutator with a plurality of brushes; a primary circuit having control field windings disposed on a first group of said poles for inducing a voltage difference between a first group of said brushes; and a plurality of internally excited amplifying stages arranged in cascade relative to one another and including a first stage having an input circuit connected to said first group of brushes for exciting a second group of said poles so as to induce a voltage difference between a second group of said brushes, and a second stage having an input circuit connected to said second group of brushes for exciting a third group of said poles to produce a voltage difference between a third group of brushes; and a secondary circuit for providing load current under control by said latter voltage difference.

2. A rotary direct-current generator, comprising a field structure with a plurality of poles; an armature having a commutator with a plurality of brushes; a primary circuit having control field windings disposed on a first group of said poles for inducing a voltage difference between a first group of said brushes; and a plurality of internally excited amplifying stages arranged in cascade relative to one another and including a first stage having an input circuit connected to said first group of brushes for exciting a second group of said poles so as to induce a voltage difference between a second group of said brushes, and a second stage having an input circuit connected to said second group of brushes for exciting a third group of said poles to produce a voltage difference between a third group of brushes; and a secondary circuit for providing load current under control by said latter voltage difference, said third group of poles being larger in number than one of said other group of poles and including said one other group of poles, and said third group of brushes being larger in number than the corresponding other group of brushes and including the latter; and a secondary circuit for providing load current under control by said latter voltage difference.

3. A rotary direct-current generator, comprising a field structure with a plurality of poles; an armature having a commutator with a plurality of brushes; a primary circuit having control field windings disposed on a first group of said poles for inducing a voltage difference between a first group of said brushes; and a plurality of internally excited amplifying stages arranged in cascade relative to one another and including a first stage having an input circuit connected to said first group of brushes for exciting a second group of said poles so as tc induce a voltage difference between a second group of said brushes, and a second stage having an input circuit connected to said second group of brushes for exciting a third group of said poles to produce a voltage difference between a third group of brushes; and a secondary circuit for providing load current under control by said latter voltage difference, said internally excited amplifying stages including a stage whose pole ratio relative to the preceding stage is at least two to one; and a secondary circuit for providing load current under control by said latter voltage difference.

4. A rotary direct-current generator, comprising a multi-pole field structure having multi-pole field windings for providing a magnetic iiux component oi balanced distribution, an armature having a commutator with a plurality of brushes disposed to assume positive and negative electric polarities respectively, controllable input means inductively coupled with said field structure for providing a first unbalanced fiux component, field exciting means connected to an equipolar group of said brushes so as to excite in said structure an amplified other unbalanced flux component due to circulating current caused by said first unbalanced flux component, said multi-pole field windings being connected to another group of equipolar brushes so as to be controlled in dependence upon amplified circulating current caused by said amplified unbalanced flux component, and circuit means inductively associated with said group of brushes to be impressed by voltage generated in said armature under control by'said fiux component of balanced distribution.

5. A rotary direct-connected generator, comprising a field structure with a plurality of field poles, an armature having a commutator with a plurality of brushes, circuit connections disposed between respective groups of said brushes to provide a plurality of paths for circulating armature currents, field control means for exciting a group of said poles to produce a circulating current in one of said circuit connections, said one circuit connection being disposed for exciting another group of said poles in order to cause an amplified circulating current to ow in another one of said connections, two field windings disposed on each of said field poles for excitation under control by said amplified circulating current, said field windings on said plurality of poles being series connected with one another so that said excitation is cumulative for the two field windings on each of said plurality of poles, and circuit means conductively associated with said plurality of brushes through said connections to be impressed by voltage generated in said armature under control by ysaid field windings, said circuit means being attached to said series connected iield windings at a point betweenvthe two neld windings o! each of said poles so that said two eld windings act differentially relative to each other as regards the flow of current through said circuit means.

6. A rotary direct-connected generator,- comprising a field structure with a plurality of field poles, an armature having a commutator with a plurality of brushes, circuit connections disposed betweenrespective groups of said brushes-to provide a plurality of paths for circulating armature current, field control means for exciting a group of said poles to produce circulating current in one of said circuit connections, said 'one circuit i8 between respective groups or said brushes to provide a plurality of paths for circulating armature current,iield control means for exciting a group of said poles to produce circulating current in one oil said circuit connections, said one circuit connection -being disposed for exciting another l group of said poles in order to cause an ampliconnection being disposed for exciting anotherA group of said poles in order to cause an amplied circulating current to flow in another one of said connections, a plurality o field windings disposed on said plurality of poles respectively and controlled in dependence upon said amplified circulating current, and circuit means conductively associated with said plurality of brushes through said connections so as to be impressed by voltage generated in said armature under control by said field windings, at least one of said plurality of connections forming a short circuit between the appertaining group of -brushes in order to excite the appertaining group of poles by armaturereaction ux, a plurality of field windings disposed on said plurality of poles respectively and controlled in dependence upon said amplified circulating current.

'7. A rotary direct-connected generator, comprising a field structure with a plurality of field poles, an armature having a commutator with a plurality of brushes, circuit connections disposed between respective groups of said brushes to provide a plurality of paths for circulating armature current, field control means for exciting a group of said poles to produce circulating current in one of said circuit connections, said one circuit connection being disposed for exciting another group of said poles in order to cause an amplified circulating current to flow in another one of said connections, at least one of said plurality of connections comprising field coils inductively associated with the appertaining group of poles and having two coils disposed on each pole to be excited .by the circulating current of said latter connection, said two coils being series connected with each other and arranged to act cumulatively relative to said latter circulating current, two field windings disposed on each of said plurality of field poles for excitation under control by said amplified circulating current, said field windings on said plurality of poles being series connected with one another so that said excitation is cumulative for the two field windings on each of said plurality of poles, and two circuit leads conductively associated with said plurality of brushes through said connections in order to be impressed by voltage generated in said armature under con' trol by said field windings, the conductive path between said leads and said brushes extending through a point between said two coils and through a point between said two field windings so that said coils and said windings are differentially arranged respectively relative to the now of current through said path.

8. A rotary direct-connected generator,` comprising a field structure with a plurality of field poles, an armature having a commutator with a plurality of brushes, circuit connections disposed tied circulating current to now in another one of said connections, said one circuit connection being disposed for exciting another group of said poles in order to cause an amplified circulating current to flow in another one of said connections, at least one of said plurality of connections comprising auxiliary coils to be excited vby the appertaining circulating current and inducti'vely associated with said leld structure for counteracting the effect of armature reaction on said field control means, a plurality 'of neld windings disposed cn said plurality of poles respectively and controlled in dependence upon said ampliiied circulating current, and circuit means conductively associated with said plurality of brushes through said connections so as to be impressed by voltage generated in said amature under control by said ileld windings.

9. A rotary direct-connected generator, comprising a field structure with a plurality o! ileld poles, an armature having a -commutator with a plurality of brushes, circuit connections disposed between respective groups of said brushes to provide a plurality of paths for circulating armature current, eld control means for exciting a group oi said poles to produce circulating current in one of said circuit connections, said one circuit connection being disposed for exciting another group of said poles in order to cause an amplified circulating current to flow in another one of said connections, at least one of said plurality of connections comprising two `auxiliary coils inducsaid brushes extending through a point between I said two auxiliary coils so that said coils act differentially relative to current traversing said path.

10. Avrotary direct-connected generator. comprising a field structure with a plurality of field poles, an armature having a commutator with a plurality of brushes, circuit connections disposed between respective groups of said brushes to provide aplurality of paths for circulating armature current, field control means for exciting a group of said poles to produce circulating current in one of said circuit connections, said one circuit connection being disposed for exciting another group of said poles in order to cause an amplified circulating current to flow in another one of said connections, at least one of said plurality of connections comprising field coils inductively associated with the appertaining group of poles, and auxiliary coils disposed in one of said connections and inductively associated with said structure for counteracting the opposing effect of armature reaction on the control function of said field coils, a plurality ofl field windings disposed on said plurality of poles respectively and controlled in dependence upon said ampli- 19 ned' circulating current, and circuit means inductively associated with. said plurality oi brushes through said connections vso as to be impressed by vo1tage generated in said armature under control by said neld windings.

11. A rotary direct-connected generator, comprising a ileld structure with a plurality of ileld poles and a corresponding plurality of inter poles, an armature having a commutator with a corresponding plurality oi' brushes, circuit connections disposed between respective groups of said brushes to provide a plurality of paths for circulating armature currents, iield control means for exciting a group of said poles to produce circulating current in one oi said circuit connections, said one circuit connection being disposed for exciting another group oi said poles in order to cause an amplified circulating current to flow in another one of said connections, a plurality oi iield windings disposed on said plurality oi' poles respectively and controlled in dependence upon said ampliiied circulating current, two circuit leads conductively associated with said plurality of brushes through said connections to'be impressed by voltage generated in said armature under control by said ileld windings, and corn-v mutation windings arranged on said inter poles and disposed in said connections so that said inter poles receive component excitation due to said circulating currents and component excitation due to current ilowing in said leads in order to vary the distribution of the resultant commutation iield in accordance with changes in operating conditions of the generator.

12. A rotary direct current generator, comprising a field structure with four field poles for developing sequentially arranged north and south polarities, an armature of substantially 90 degree chording having a commutator with four brushes arranged to assume sequentially positive and negative electric polarities, control iield windings disposed on said structure for varying the relative strength of two equipolar poles in order to produce a corresponding difference in potential between two equipolar brushes, circuit means connected between said two brushes and disposed for varying the relative field strength. of said other two poles due to circulating current caused by said potential difference, two ileld windings disposed on each of said i'our poles and connected in series between said two other brushes to be cumulatively excited by circulating current due to said strength variation of said other two poles. and two circuit leads attached to said connection and to a point between said two field windings.

13. A rotary direct current generator, comprising a ileld structure with four ileld poles ior developing sequentially arranged north and south polarities, an armature of substantially 90 degree chording having a commutator` with four brushes arranged to assume sequentially positive and negative electric polarities, control iield windings disposed on said structure for varying the relative strength of two equipolar poles in order to produce a corresponding difference in potential between two equipolar brushes, a short-circuit connection disposed between said two brushes for exciting said two other poles -by armature reaction, two ileld windings disposed on each of said four poles and connected in series between said two other brushes to be cumulatively excited by 20 circulating current due to armature-reaction excitation, and two circuit leads attached to said connection and to a point between said two neld windings.

14. A rotary direct current generator, comprising a ileld structure with four ileld poles for developing sequentially arranged north and south polarities, an armature of substantially degrees chording having a commutator with four brushes arranged to assume sequentially positive and negative electric polarities, control ileld windings disposed on said structure for varyingl the relative strength oi two equipolar poles in order to produce a corresponding difference in potential between two equipolar brushes, tour exciting coils series connected between said two brushes. two of said coils being arranged on each of said other two poles so as to be cumulatively excited by circulating current caused by said difference in potential, eight eld windings series connected between said Vtwo other brushes, two of said ileld windings being arranged on each of said four poles so as to be cumulatively excited by circulating current due to the excitation oi said two other poles. and two load circuit leads attached to the midpoint oi said four coils and the midpoint of said eight windings respectively so that said coils and said windings. relative to said individual poles, are differentially connected with respect to the flow of current in said leads.

l5. A rotary direct current generator, comprising a neld structure with fourileld poles for developing sequentially arranged north and south polarities, an armature oi substantially 90 degrees chording having a commutator with four, brushes arranged to assume sequentially positive and negative electric polarities, control ileld windings disposed on said structure for varying the relative strength of two equipolar poles in order to produce a corresponding diierence in potential between two equipolar brushes, a circuit connection attached to said two brushes and disposed for exciting said other two poles due to circulating current caused by said potential diierence, another circuit connection extending between said two other brushes and containing eight series connected neld windings of which two are arranged on each of said four poles respectively to be cumulatively excited by circulating current due to the excitation of said two other poles. a load circuit attached to said two connections so that said two windings of each pole are differentially arranged relative to said load circuit, and at least one pair of balanced compensating coils arranged on said structure for counteracting the eiiect of armature reaction on said control eld windings and disposed in one of said connections so as to be cumulatively excited by circulating current flowing in said connection while cancelling the exciting eiTect of current traversing said load circuit.

MICHAEL LIWSCHITZ. ALBERT W. KIMBALL.

REFERENCES CITED UNITED STATES PATENTS Name Date Rosenberg Apr. 12, 1910 Number 

